The following terms will be used in this document: A “treating fluid” or “treatment fluid” is a fluid that is used for treating a well. In this particular case, it is generally a water shutoff fluid. It should be understood that the same materials and techniques may generally be used for water shut off and for gas shut off, either or both of which are called “conformance control,” and that when we speak of water shut off compositions and methods we intend gas shut off to be included. In the case of a delayed water shut off treatment fluid that contains colloidal silica and urea, the “delayed water shut off gel” is a gel formed by the delayed water shut off treatment fluid. For example, a delayed water shut off treatment fluid that contains colloidal silica and N,N′-dimethyl urea according to the present invention will typically form a hard gel in about 1 to about 20 hours (this is called the “working time” and depends upon such factors as the component concentrations and the temperature).
In subsurface formations, naturally occurring rocks are generally permeable to fluids such as water, oil, or gas (or some combination of these fluids). It is believed that the rock in most oil-bearing formations was completely saturated with water prior to the invasion and trapping of petroleum. The less dense hydrocarbons migrated to trap locations, displacing some of the water from the formation, the trap becoming a hydrocarbon reservoir. Thus, reservoir rocks normally contain both petroleum hydrocarbons (liquid and gas) and water. Sources of the water in a reservoir may include flow from above or below the hydrocarbon zone, flow from within the hydrocarbon zone, or flow from injected fluids and additives resulting from drilling, completion, and/or production activities. The naturally occurring water is frequently referred to as “connate water” or “formation water” and becomes produced water when the reservoir is produced and these fluids are brought to the surface. Produced water is any water that is present in a reservoir with the hydrocarbon resource and is produced to the surface with the crude oil or natural gas. When hydrocarbons are produced, they are typically brought to the surface as a produced fluid mixture.
As production continues, it is common that an increasing proportion of the produced fluids is water. There are some strategies that can be used to restrict water from entering the well bore. These involve mechanical blocking devices or chemicals that “shut off” water-bearing subterranean formations, channels, or fractures within the formation and prevent water from making its way to the well. Operators have used various mechanical and well construction techniques to block water from entering the well. Several examples of these techniques are straddle packers, bridge plugs, casing patches, and cement plugs (or cement squeezes).
These techniques have been used for many years, but they do not work well in all applications. Mechanical approaches can be used to block casing leaks, or flow behind the pipe without flow restrictions, and to block unfractured wells with barriers to cross flow. However, these approaches may not be effective in solving other types of water production problems. Another drawback of these mechanical methods is the physical restriction left in the wellbore. This restriction can, in some cases, prevent the subsequent perforation or the mechanical isolation of an interval located below the treated interval.
Another approach to shutting off unwanted water or gas production while allowing continued production of oil or gas involves the use of gel-based shut-off fluids. These fluids are introduced deep into the pore matrix of the formation that is producing unwanted water or gas, and into the channel or fracture network, where they undergo physical transformation from a solids free squeezable liquid to a highly viscous or rigid material. Two families of gel-based shut-off fluids are typically used. Polymer gels typically contain an acrylamide gel and a cross-linker. The physical transformation occurs when the polymer is cross-linked. This process is triggered by time and/or temperature and can be delayed to allow sufficient time for placement in the target formation. Inorganic gels typically contain a metallic or silicate salt and an activator. The physical transformation occurs when the pH of the solution is modified by chemical reaction of the activator. This process is also triggered by time and/or temperature and can also be delayed to allow sufficient time for placement into the target formation.
Gel-based shut-off treatments are typically formulated to “set” by precipitation or cross-linking after several hours so that enough time is available for the treatment to be pumped and squeezed into the target formation. This delay time is also known as working time.
Background information on conformance control of a petroleum reservoir with a delayed gel can be found in Borling et al., “Pushing Out the Oil with Conformance Control,” Oilfield Review, p. 44 (April 1994). This reference discusses conformance control with, among others, (i) the DGS™ (Trademark of Schlumberger Technology Corporation) inorganic gel system, based on aluminum hydroxyl chloride-urea, (ii) the MARA-SEAL™ (Trademark of Marathon Oil Corporation) polymer gel system, based on partially hydrolyzed polyacrylamide-chromium acetate, and (iii) the BP/ARCO gel system based on partially hydrolyzed polyacrylamide-aluminum citrate.
General background information on water shut off with polyacrylamide and other polymers is seen in U.S. Pat. Nos. 5,125,456 and 5,957,203, both to Hutchins et al., U.S. Pat. No. 4,039,029 to Gall, U.S. Pat. No. 5,382,371 to Stahl et al., and U.S. Pat. No. 5,010,954 to Falk.
General background information on inorganic gels is found in U.S. Pat. No. 4,889,563 to Parker et al. which teaches a process for retarding and controlling the formation of gels or precipitates from aluminum hydroxyl chloride with a delayed activator such as urea or hexamethylene-tetramine, optionally with gelling additives such as citrate and tartrate and/or crystallizing additives such as sulfate, oxalate and succinate.
Conformance control with silica sols is generally described in U.S. Pat. No. 4,732,213 to Bennett et al. (Conoco). A method of completing poorly consolidated formations with an aqueous solution of colloidal particles is described in U.S. Pat. No. 7,013,973 to Danican et al. (Schlumberger). General background information on the testing of colloidal silica gels may be found in Jurinak et al., “Laboratory Testing of Colloidal Silica Gel for Oilfield Applications,” SPE Paper No. 18505, 1991.